The Grand Jury is composed of twelve citizens who are chosen at random by a circuit judge, who presides over the Grand Jury. A new Grand Jury is chosen for each of the three Terms of Court held each year. Since the St. Louis County Grand Jury meets every Wednesday during the term, a Grand Juror can expect to serve approximately sixteen to twenty days during that period.

The Grand Jury's function is to make an important preliminary decision about a criminal charge: whether or not probable cause exists that a crime has been committed and that the accused committed it. All felony charges must be presented for consideration either by the Grand Jury or by an associate circuit judge at a preliminary examination, and a little less than half of the cases prosecuted in St. Louis County are presented to the Grand Jury. If probable cause is found in either of these equivalent proceedings, a "True Bill" is found, an indictment is filed, and the criminal charge against the accused may continue on to trial or plea of guilty. If probable cause is not found, a "No True Bill" is found, and prosecution of the case ceases. The Grand Jury, thus, determines the future of all criminal cases brought before it.